Rotating components used in jet engines and other high-speed machineries operate under large centrifugal stresses and can be fatigued through repeated use. For example, the Federal Aviation Administration (FAA) requires testing of newly-designed and revised engine hardware to establish life expectancy during the development phase of a new engine, and also when significant changes are made to an engine design.
Most jet engine manufacturers spend substantial time and money on computer simulations (“finite element models” of the engine hardware) to obtain an initial estimate of the safe operating life of an engine part. It is impossible, however, to determine a rotor's actual characteristics until it has been built and tested. Centrifugal fatigue life is generally measured at a centrifugal stress testing facility, in a spin test system designed to cycle the rotor from some low speed to operational speed then back again, alternately applying and relaxing the centrifugal stress.
Jet engines have numerous rotating parts that move and compress air (fans and impellers), or produce work (turbines). The elevated speeds at which these parts rotate induce high levels of centrifugal stress that tend to pull the components apart. A jet engine part such as a rotor usually fails in one of two ways. In the first failure mode, the rotor rotates to a speed that is sufficient to cause catastrophic material failure or burst. However, even when a rotor rotates at less than its burst speed, the rotor may eventually weaken over time as a result of many starts and stops. In this second failure mode, the part fatigues to a point where it develops a crack, which then grows to a critical size and ultimately causes the part to fail.
Typically, jet engine components such as rotors are thoroughly tested by the manufacturer as part of a development and qualification process to establish a safe operating life. The manufacturer will generally use a type of spin testing known as “fatigue life” testing. Fatigue life is measured in cycles, with a run up to operating speed and back down to zero or some lower speed being counted as one cycle. Each cycle corresponds roughly to one takeoff and landing of an aircraft. After the designer has measured the number of cycles a part can withstand before a fatigue burst happens, safety and performance factors can be developed and applied. The safety factor determines how many cycles can be tolerated by an engine before a part must be replaced. The safety margin is established cooperatively by the engine manufacturer and the appropriate governing safety authority, and it is intended to assure that parts are replaced before there is any chance of burst in the engine.
Jet engine rotors are also routinely subject to periodic inspection after installation to determine the health of the rotor. To inspect an installed rotor, the engine is taken apart and inspected with fluorescent penetrant, or inspected with eddy-current type crack probes. Moreover, methods are known for evaluating the health of a rotor by electronically monitoring vibrations. These methods generally measure broad-band vibration and infer the existence of problems when there is an overall increase in vibration amplitude. Other techniques are known as well, such as the method described in U.S. Pat. No. 4,751,657, to Imam et al. The method described in this patent uses changes in synchronous vibration as a function of speed to evaluate rotor health. Establishing safe operating component lives is a critically important process, since the fragments of a bursting rotor cannot be contained by the engine casing. A rotor burst in flight would probably destroy the aircraft. The air transport industry has achieved its admirable safety record due in no small way to spin-pit life testing of engine parts; still, there have been some tragic accidents in air transport due to rotor burst. Examples of accidents traced to fatigue failure include the DC-10 crash at Sioux City; the in-flight separation of a propeller blade in the crash of an EMB-120 Embraer near Carrollton, Georgia; and the fatal explosion of a fan disk assembly during take off of an MD-80 in Florida.
There is, therefore, still an unmet need for a technique which can accurately detect fatigue, cracks, and other anomalies in rotating components such as jet engine rotors and which is less cumbersome to use than penetrant or eddy current or other known techniques. Ideally, the technique could be used in a centrifugal spin testing facility used during engine qualification as well as for in-flight instrumentation which might continuously monitor the health of a jet engine.